Ink-jet recording aqueous inks have become the major ink for ink-jet recording use other than an industrial use because the danger of fire and toxicity such as mutagenicity, which are peculiar to oil-based inks, can be reduced.
In such aqueous inks, dyes have been used as coloring agents because stability is high, nozzle clogging is reduced, color-developing property is favorable, and printing with high image quality is possible. However, dyes have the problems of poor water resistance and light fastness.
In order to solve these problems, the conversion of coloring agents from dyes to pigments has been actively carried out. Pigment inks are expected to have excellent water resistance and light fastness, but the problem of nozzle clogging occurs due to the aggregation/precipitation of pigments. For this reason, various methods have been investigated, which use polymer-based dispersants to disperse microparticulated pigments in an aqueous medium.
In order to carry out multicolor printing using these pigments as coloring agents, all of the 4 colored inks, i.e. black, cyan, magenta, and yellow, have to fulfill, in addition to color-developing property, good dispersion stability, discharging property, and preservation stability.
Therefore, the selection of a pigment that is appropriate for each color, the combination of a pigment and a polymer dispersant which can favorably and stably disperse each pigment, and a production method of an aqueous pigment liquid dispersion using the aforementioned combination have been considered in detail. However, there are numerous types of pigments corresponding to each color, and the details of a dispersion method to be used vary according to respective pigments. Therefore, the optimal ink-jet recording inks for all colors are not necessarily obtained so far.
In recent years, the attempts have been carried out, in which color reproduction of a printed image is improved by using inks having color tones such as red, green, and violet, in addition to the 4 colored inks of black, cyan, magenta, and yellow. Moreover, the ink-jet recording pigment inks have been desired, in which a lot of colors fulfill, in addition to good color-developing property, good dispersion stability, discharging property, luster of an image, and preservation stability.
When the 3 colors of red, green, and violet are combined with the aforementioned basic 4 colors, it is found that an image having extremely enlarged color zone and good color reproduction can be formed by using pigments having preferable color-developing zone (see Patent Reference 1).
However, it was difficult to select pigments having both ideal color-developing zone and the aforementioned ink-jet adequacy, and to use the pigments so as to provide ink-jet recording inks simultaneously having dispersibility, discharging property, and preservation stability.
Regarding green color for example, the investigation has been carried out on the special ink-jet recording ink for formation of a color filter pattern as an ink-jet recording ink that uses a green pigment. However, almost all of conventional inks for formation of a color filter pattern are organic solvent-based inks. In the case where the formation of a color filter pattern is carried out using ink-jet recording inks, a special ink-jet printer in an air-conditioned factory is operated according to operating time of production facilities. However, in order to be widely used for a consumer ink-jet printer, the ink-jet recording inks need to directly respond to printing request as needed and to provide a stable printed image even after long-term preservation in an ink container. The temperature range of an environment in which the ink-jet recording ink is used by a consumer is wide. Therefore, in order that these ideal operations are performed by an inexpensive consumer ink-jet printer, the ink-jet recording inks have to keep very good dispersibility and stable discharging property.
In recent years, the demand for long-term stability of color-development of a printed image has increased. Moreover, as the case where printed matter is industrially used outside has increased, good light fastness of ink-jet recording pigment inks has simultaneously been demanded. In addition, when inks are expected to be used as an ink for thermal jet recording, preservation stability at high temperature is essential. As described above, the demands for the properties of inks have been strict.
In order to fulfill the aforementioned demands, extensive studies are needed on the selection of a green pigment, the selection of a polymer dispersant to be combined therewith, the optimal composition when the dispersant is used, and the production method that uses the composition.
In addition, the number of types of green pigment is very small in comparison with basic 4 colors. Therefore, by using a small number of green pigments, all of the necessary properties of consumer ink-jet recording inks should be realized such as light fastness, color-developing property, and dispersion stability. However, the study for improving the basic properties of the ink-jet recording ink that uses a green pigment is not sufficient, and currently, the same levels of dispersibility and preservation stability as those of inks that use basic 4 colors are not realized in the ink-jet recording ink that uses a green pigment.
In addition to pigment green 7, C. I. pigment green 36 is often used as a green pigment for various uses. Of these, C. I. pigment green 36 has so good color-developing property and light fastness that it can be used as a color filter. However, C. I. pigment green 36 contains a lot of bromine atoms and has a high specific gravity. Therefore, C. I. pigment green 36 is likely to precipitate in a liquid, and it is very difficult to maintain dispersion with a low viscosity that is typical in an ink-jet recording ink. Moreover, the aggregation of the pigment particles is likely to occur. For good dispersion, it is essential to perform the adsorption of polymer dispersants to the pigment surface and to perform the stabilization of dispersion due to steric hindrance.
For example, there is the report on the method of coating C. I. pigment green 36 with the copolymers that contain, as essential components, styrene monomers of 65 to 85 mol % at a monomer ratio, acrylate monomers of 5 to 15 mol %, and methacrylate monomers of 10 to 20 mol %, to thereby produce an ink-jet recording ink having good light fastness and dispersibility (see Patent Reference 2). However, in the ink-jet recording ink produced by the aforementioned method, the molecular weight of the used resins is high, and the resins are used at the almost same quantity as that of the pigments in order to stably disperse C. I. pigment green 36. Because the quantity of the used resins is not reduced sufficiently, the discharging property in a high-temperature environment and the stable discharge after long-term preservation are not sufficient although the initial dispersibility is good.
Moreover, there is the disclosure of the ink composition in which C. I. pigment green 36 is used as a green pigment within a 6 colored ink set and the styrene-acrylic acid type copolymers with a molecular weight of 10,000 are used as a dispersant resin (see Patent Reference 1). However, the pigment is not strongly coated with the resins because the ink composition is produced only by the dispersion using a dispersing machine with a media. The quantity of the resins used for the stabilization of the pigment dispersion is large, and the large quantity of resins that are not directly related to the coating of the pigment is present in an aqueous medium. Therefore, the discharging property in a high-temperature environment and the long-term preservation stability during preservation are not particularly sufficient.
Moreover, there is the disclosure of the ink-jet ink in which C. I. pigment green 36 is used as a pigment, the copolymers with a weight average molecular weight of 11,000 and an acid value of 325, which are made of styrene/methacrylic acid/butyl acrylate, are used as a dispersant, and the ratio of the dispersant to the resin is reduced to 1:4.5 (see Patent Reference 3). However, the aqueous solution, in which the copolymers are dissolved with potassium, is prepared in the production of the ink, and C. I. pigment green 36 is dispersed in the aqueous solution. Therefore, the resins are not sufficiently attached to the pigment surface, and the existence of the resins dissolved in the ink medium may deteriorate the preservation stability during high-temperature preservation.
In addition, there is the report on the aqueous pigment dispersion in which water-soluble phthalocyanine derivatives adsorb to the surface of the pigment particles of C. I. pigment green 36, to thereby improve the dispersibility of the pigment and reduce the quantity of the added acrylic water-soluble resin solution such that the mass ratio of the acrylic water-soluble solution to the pigment is reduced to about 1/10 (see Patent Reference 4). However, the water-soluble phthalocyanine derivatives are very likely to elute into the dispersion, and cause the deterioration of the long-term preservation stability of the aqueous pigment dispersion together with the resin component originally dissolved in the dispersion.
As described above, the several known documents disclose the aqueous pigment liquid dispersion and the ink-jet recording ink that use C. I. pigment green 36 as a green ink that should be added in the ink set of the basic 4 colors because the pigment green 36 has good color-developing property and light fastness. However, C. I. pigment green 36 has a high specific gravity and is likely to precipitate. Therefore, in order that the pigment green 36 is stably dispersed to obtain the same properties as those of the basic 4 colors, it is necessary to prepare the aqueous pigment liquid dispersion and the ink-jet recording ink by using different composition and production steps from those of the basic 4 colors.
In order to particularly improve the discharging stability and preservation stability in a high-temperature environment, it is important to strongly coat the surface of C. I. pigment green 36 with the required minimum resins, to thereby reduce the quantity of the resins dissolved or dispersed in an aqueous medium. Regarding the ink-jet recording ink disclosed in the aforementioned Patent References, the various types of dispersant and additives are used to aim at the improvement of dispersion stability, but these Patent References are not necessarily focused on the case where C. I. pigment green 36 is used. As a result, the ink-jet recording ink of green color has never had dispersion stability and preservation stability that are as sufficient as those of inks of the other basic colors. In particular, the aqueous pigment liquid dispersion and the ink-jet recording ink, which have practically sufficient dispersion stability and preservation stability during high-temperature preservation, have never been obtained.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. 2001-354886
[Patent Reference 2]
Japanese Unexamined Patent Application, First Publication No. 2000-186244
[Patent Reference 3]
Japanese Unexamined Patent Application, First Publication No. 2002-088290
[Patent Reference 4]
Japanese Unexamined Patent Application, First Publication No. 2000-303014